1. Field of the Invention
The present invention relates to an apparatus for detecting the positions of the wheels of a vehicle and an apparatus for detecting tire inflation pressure using such a vehicular wheel position detecting apparatus.
2. Description of the Related Art
One of known apparatuses for detecting inflation pressure of each tire of a vehicle is a direct detection type of apparatus for detecting the tire inflation pressure. This direct detection type of tire inflation pressure apparatus is provided with transceivers directly mounted to wheels to which tires are secured. The sensors are for example pressure sensors. In addition, an antenna and a receiver are provided in the vehicle body, so that the transceiver transmits, to the receiver via the antenna, a pressure detection signal detected by the sensor. Thus the receiver is able to detect the tire inflation pressure of each tire.
In this direct detection type of tire inflation pressure detecting apparatus, it is significant that the apparatus is able to determine whether or not detected pressure data is from the vehicle itself and determine that which transceiver is mounted to which wheel.
Considering such respects, Japanese Patent Laid-open Publication No. 2007-15491 disclose a wheel position detecting apparatus provided with a triggering device located to have different distances from transmitters (serving as transceivers) mounted to the wheels. In this configuration, the fact that a triggering signal outputted from the triggering device decays depending on the distances from the wheels is used to determine that which transmitter is mounted to which wheel. More specifically, the triggering device transmits a triggering signal and each transceiver receives the triggering signal to measure the strength of the received triggering signal. Data indicating the measured signal strength are then transmitted to the receiver provided on the vehicle body. The receiver applies processes to the data to specify the transceiver positions, i.e., the wheel positions.
However, in such a configuration, when the triggering signal is lower in strength than noise, each transceiver cannot receive the triggering signal in good order. In particular, when the noise strength is higher, there is a risk that a transceiver located farther from the triggering device cannot receive the triggering signal at all. In such a case, the transceiver cannot notify the receiver of the data showing the strength of the triggering signal.
If such an occasion happens, the receiver tries in succession to urge the triggering device to output the triggering signal until the receiver receives strength data from the transceiver. When there is provided no strength data from the transceiver even if the triggering signal has been outputted in succession for a certain period of time, the receiver finally decides that there occurs a system malfunction in the apparatus. Hence, a notice for the system malfunction is given to the driver using an alarm, for example. The system malfunction can be defined as malfunctions due to a failure, accident or others derived from the system itself that composes the apparatus. This term “system malfunction” is used to distinguish malfunctions occurring due to external factors such as external noise.
However, the above alarming system is still confronted with a difficulty. Even when a higher-strength noise interferes with the correct reception of the triggering noise at the transceiver in a strongly noisy environment, it is possible to normally detect the wheel positions after the vehicle has passed through the strongly noisy environment, because the apparatus itself has no system malfunction from the beginning. Nonetheless, the apparatus erroneously determines a system malfunction occurring in the apparatus and outputs an alarm, which results in issuing an error alarm.